<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .bar {
            width: 400px;
            height: 50px;
            border: 1px solid red;
            border-radius: 25px;
            overflow: hidden;

        }

        .in {
            width: 0px;
            height: 50px;
            background: green;
            animation: 5s move;
        }

        @keyframes move {
            0% {}

            100% {
                width: 400px;
            }
        }
    </style>
</head>

<body>
    <div class="bar">
        <div class="in"></div>
    </div>
</body>

</html>